The IEEE 802.11 protocol provides support for two different medium access control (MAC) mechanisms that may be utilized for transporting asynchronous and time bounded services. The first mechanism is distributed coordination function (DCF) and the second is point coordination function (PCF). The distributed coordination function utilizes best effort for facilitating communication of information in which access devices with information to transmit have an equal opportunity to transmit information. The point coordination function may be utilized to communicate time sensitive or latency sensitive information. In this regard, the point coordination function utilizes a polling mechanism, which may be controlled by an access point (AP).
The standard distributed coordination function for medium access may be inefficient in terms of bandwidth utilization, especially at higher physical layer (PHY) speeds, for example, 54 Mbps or higher. The DCF may be adapted to solve problems such as network congestion and high packet error rate (PER) typically associated with some wireless links. The DCF may also exponentially increase backoff and positive acknowledgments (PACKs). The backoff time for each MAC protocol data unit (MPDU) may increase exponentially for retransmissions and the PACK for each MPDU may render bandwidth utilization inefficient at high physical layer (PHY) speeds. Furthermore, When DCFs are utilized without request to send (RTS)/clear to send (CTS) acknowledgement signals, it may not be possible to distinguish between collisions and errors. As a result, larger backoffs than necessary are required. The RTS/CTS mechanism when utilized in conjunction with regular DCF, may diminish efficiency even more, and as a result, may be rarely utilized. For example, in a case where no RTS/CTS is utilized, transmitting a 1500 byte frame including MAC header at 54 Mbps takes 248 μs. The sum of the average backoff, PACK and the short interframe space (SIFS) takes 130 μs, when PACK is transmitted at 24 Mbps. The overhead air time may be more than half the data air time.
The distributed coordination function may not be the most bandwidth efficient transport mechanism. The IEEE 802.11 standard defines a bursting method for MAC protocol data units (MPDUs), called fragmentation. In this regard, MAC service data units (MSDU) may be fragmented at the MAC level to a number of smaller MPDUs. The individual MPDUs comprising one MSDU may be transmitted in a “burst”, in which the interframe spacing is a SIFS. Hence, a typical frame exchange sequence under fragmentation would be DATA-SIFS-PACK-DATA-SIFS-PACK, for example, with an optional RTS/CTS exchange in the beginning. However, fragmentation was defined in the 802.11 standard as a means to combat unreliable wireless links having high packet error rates (PER). In reliable wireless links, that is, those with low packet error rates, fragmentation may decrease MAC efficiency, since it introduces MAC headers on each MPDU, and SIFS intervals between MPDUs.
The IEEE 802.11e draft standard defines block acknowledgement policies that eliminate the need for individual acknowledgements (PACKs) for each MPDU. However, this block PACK mechanism introduces extra overhead comprising block PACK request frames and block PACK response frames. The IEEE 802.11e draft standard also defines contention free periods of time allocated to specific devices, where frames may be transmitted with a SIFS period. The mechanism for this allocation may be complex, and may include overhead for the polling mechanisms involved.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.